There is a growing interest in renewable energy technologies throughout the world. For instance, climate change concerns are driving energy production to renewable energy technologies. Wind power is therefore an important energy source and the amount of power produced annually through wind power is growing rapidly.
Wind power is the conversion of wind energy into more useful forms, such as electricity. In this regard, use is made of a wind turbine, which is a device that converts kinetic energy from the wind into electrical power. A wind turbine comprises a rotor having a central hub, to which one or more blades are attached. The rotor is arranged to rotate as the blades are subject to a mass of air passing the wind turbine due to a blowing wind. The rotation of the rotor thus generates mechanical energy that may be converted to electrical power in the wind turbine.
There are two main types of wind turbines, horizontal-axis wind turbines (HAWT), wherein the blades rotate about a horizontal axis, and vertical-axis wind turbines (VAWT), wherein the blades rotate about a vertical axis. The far most common type of wind turbine for large-scale power production is the HAWT and the discussion below is mainly directed to HAWTs.
The blades are formed with an airfoil-shaped cross-section. This implies that the blades are formed such that the surface at the leading side in the rotational direction of the blade causes the air passing the surface to take a longer path than the air passing the surface at the trailing side. Hence, the air passing over the surface at the leading side will travel faster than the air passing over the surface at the trailing side. Therefore, a difference in pressure is formed, resulting in a force on the blade. This force induces a torque about a rotor axis which causes the rotor to rotate.
The relative flow velocity, including speed and direction, between a moving blade and the air is called the apparent flow velocity. As air passes the surface of an airfoil shaped blade it exerts a force on it that depends on the apparent flow velocity and the shape of the airfoil. Lift force is the component of the force that is perpendicular to the oncoming apparent flow direction. It contrasts with the drag force, which is the component of the force parallel to the apparent flow direction. Contrary to the lift force, the drag force tends to counteract the movement of the blade and it can be shown through mathematical analysis that in order to optimize the power efficiency of the turbine the blade should be designed so as to maximize the ratio between the lift force and the drag force.
The power production capacity of a wind turbine is mainly affected by the length of the blades. The power generated by a wind turbine is proportional to the area swept by the blades, which is proportional to the square of the length of the blades. Hence, an increased length of the blades enables an increased power production of the wind turbine.
However, the blades also need to be designed with the loads encountered by the blades during operation of the wind turbine in mind. Aerodynamic loads are formed by means of the apparent flow velocity of the air. The aerodynamic loads cause a bending moment on the blade, which is largest closest to the hub. While the aerodynamic loads may vary due to wind speed, the aerodynamic loads exerted on the blades are also proportional to the square of the length of the blades.
The blades are also exerted to gravity loads due to the mass of the blade, and as the blade rotates a full circle the blade will go through a fatigue cycle. The gravity loads are proportional to the cube of the length of the blade. Therefore, although the aerodynamic loads are dominating for small-size blades, the gravity loads will become dominating as the length of the blades increases.
Thus, as the length of the blades is increased in order to increase the power production capacity of wind turbines, the blades need to be designed with a close attention to the gravity loads exerted on the blades. Otherwise, there is a risk of fatigue failure due to the large mass of the blade. Furthermore, long blade will lead to problems concerning deformations, cracks and torsion of the blades. Hence, design of the blade becomes difficult as the length of the blade increases.
The mass of the blade and the associated gravity loads, as well as the aerodynamic loads, may force the design of the shape of the blade to be a compromise between strength and aerodynamics. In particular close to the hub, the blade may need to have a design which is optimized for providing strength rather than airfoil characteristics, which implies that the aerodynamic properties of the blade will not be optimal.
Further, when the wind turbine is to be installed, the wind turbine parts need to be transported to the site of the wind turbine. The wind turbine consists of very large parts, such as the long blades, which makes transportation of the parts to the site a difficult task. For instance, the wind turbine parts may be much longer than the usually allowed length of vehicles, which implies that special vehicles need to be used for transportation of the parts on land. Further, the mass of the wind turbine parts may also set special requirements in order to allow transportation of the parts to the site. Altogether, problems associated with transportation of long blades will limit the economically feasible size, at least for land based wind turbines. Also, installation of the parts on the site is cumbersome due to the mass and length of the parts.
Also, the cost of the blade of course increases with the mass of the blade. Since the mass of the blade is proportional to the cube of the length of the blade, the costs of manufacturing a blade increases more rapidly with the length of the blade than the power production capacity of the wind turbine.
Finally, a large mass of the blade may cause problems with tower and foundation of the wind turbine, as large loads are exerted on these parts of the wind turbine by the mass of the blade. Also, the increased mass of the blade causes increased loads on the rotor hub by means of the increased rotational inertia.
It is clear from the above that any modification of blades of wind turbines, such that the mass of the blades is decreased would significantly improve problems faced in design of the blades.
In U.S. Pat. No. 7,517,198, a lightweight wind turbine blade is disclosed. The turbine blade comprises a lightweight composite support truss structure. The support truss structure is covered by an assembly of skins forming the basic airfoil shape of the blade. A series of laterally spaced ribs form a spine of the blade and define the general airfoil shape. However, the blade needs to be thin in order to keep the aerodynamic loads down. This implies that it is difficult to obtain a strong structure. Therefore, the ribs closest to the hub have a circular shape providing strength to the structure rather than good aerodynamic properties.
In EP 1 887 219, a special blade structure is disclosed. The blade structure makes use of the fact that the moment of inertia of a blade can be increased by designing a profile section of the blade so as to increase the surface of the section and the distance of the section to a neutral line. Further, the stress that a material of a section in the structure supports is inversely dependent to the moment of inertia, whereby increasing the moment of inertia decreases the stress of the material. Hence, by means of dividing the blade into sub-blades and separating the sub-blades, the moment of inertia may be increased without increasing the weight of the material. However, to achieve the greater moment of inertia, the sub-blades need to be firmly joined. Therefore, links are spaced out along the length of the sub-blades. Although this structure allows the stress that a material of a section in the structure supports to be decreased, the weight of the blade is in principle not decreased. Hence, there may still be a need to decrease the weight of blades. Also, the sub-blades are exerted to bending moments, which implies that the sub-blades closest to the hub needs to be designed with regard to providing strength to the sub-blade rather than aerodynamic properties.
U.S. Pat. No. 1,820,529 discloses a propeller blade provided with a plurality of aero-foils, that merge at an apex. Each aero-foil slopes outward towards a supporting end where it is rigidly fixed to a common blade axis. A plurality of shelves are positioned to be clamped between the plurality of aerofoils, substantially parallel to the blade axis. Also, an oblique tie structure is rigidly secured to cross-brace one aerofoil with respect to the others of a given blade along the length thereof.